Heater core bad?

What are the chances my heater core is letting coolant into the ventilation system? Symptoms:
1) windshield and window insides sometimes get covered in a light film of something that's not water
2) coolant seems to be slowly disappearing from radiator overflow tank (but not leaking onto garage floor)
3) heater works just fine, I think (no other car to compare it to)
4) car interior smells vaguely sweet (could be normal?)

What options do I have other than add coolant, wipe down windows, repeat?